This invention is an apparatus for optical measurement and digital display of hole diameter on thin boards such as printed circuit boards.
Holes on printed boards are ordinarily measured with pin gauges, with a hole gauge, or with a projector.
By the first method, a number of pins with different outside diameters by 0.01 mm each other are inserted into a hole one by one from the smallest to larger ones, and the O.D. of the pin which fits snugly into the hole is taken as the diameter of the hole.
Besides the time required, this method tends to indicate hole diameters to a larger size due to wear by repeated insertion of different pins before the right one is found.
By the second method, a hole gauge (as shown in FIG. 1) is prepared and the measuring rod (1) is inserted into a hole (3) on a printed board (2) to the limit as illustrated, then the stopper (4) comes in contact with the surface of the printed board (2) pushing down the whole printed board. The pointer indication on the dial gauge is read under this condition.
This measure is disadvantageous in that the measuring rod (1) is easily broken in measuring small holes, that skill is required because the measuring rod (1) must be inserted vertically into holes to avoid errors, and that the measuring accuracy is lowered little by little due to wear of the measuring rod (1).
By the projector method, the image of a hole is enlarged and projected onto a screen in order to read the diameter on the scale graduated on the screen.
Disadvantages of this method are errors in visual reading of the scale and longer time required for the measurement.